202 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 35. 



fascicular cambium, the interfascicular cambium in the primary 

 medullary rays giving rise only to conjunctive tissue; thus the 

 primary medullary rays persist as broad bands of conjunctive 

 tissue between the bundles, and are not broken up, as is usually 

 the case, by the formation of secondary bundles by the interfasci- 

 cular cambium. 



A certain amount of secondary growth, independently of the cambium, takes 

 place in some cases in the outer portion, external to the cambium, of the primary 

 medullary rays of stems growing in circumference. In all cases the effect of 

 growth in circumference is to tend to stretch the cells in a tangential direction. 

 In the cases under consideration (e.g. Tilia, Fig. 149, p. 198) the cells of the outer 

 portion of the primary medullary rays yield to this tension more than the rest 

 of the tissue, and also undergo radial division, thus the bast-portions of the 

 bundles come to be separated by considerable areas of conjunctive tissue. 



The Differentiation of the Secondary Tissues. The cells, formed 

 as the result of division in the cambium, which are to become 

 transformed into secondary permanent tissue are (apart from the 

 primary medullary rays) collectively termed secondary desmogen 

 (see Fig. 145). They have, to begin with, the same form and 

 structure as the corresponding cambium-cells (see p. 181), but they 

 gradually undergo changes in both respects, as they become trans- 

 formed into permanent tissue. 



The development of the desmogen-cell into one or other of the 

 various forms of permanent tissue, already described, may be 

 either accompanied or unaccompanied by cell-division. In the 

 former case, the divisions may be transverse or longitudinal ; the 

 desmogen-cell undergoes transverse division when the product is 

 a row of short cells (e.g. wood-parenchyma, Fig. 153 D, and Fig. 

 148 (7; bast-parenchyma; secondary medullary rays; wood- 

 vessels with short segments) : the desmogen-cell generally under- 

 goes longitudinal division once or twice, by tangential walls, soon 

 after it has been cut oif from the cambium (Fig. 145) ; but this 

 does not take place in the line of the medullary rays, where the 

 radial diameter of the young cells is greater than it is near the 

 bast or the wood ; again, the desmogen-cells may undergo longitu- 

 dinal division in a plane other than the tangential, as for instance 

 the longitudinal division of the mother-cell, which separates the 

 sieve-tube-segment from the companion-cell in the bast of Angio- 

 sperms. 



The product of a desmogen-cell may have much the same form 

 and size as the desmogen-cell (e.g. small medullary rays ; rows of 



